Magnetic recording medium

ABSTRACT

A MAGNETIC RECORDING MEDIUM HAVING A MAGNETIC RECORDING LAYER IN WHICH IS DISPERSED IN A BINDER A FERROMAGNETIC POWDER COMPRISING A MIXTURE OF AN IRON OXIDE POWDER AND A CHROMIUM DIOXIDE POWDER IN A WEIGHT RATIO OF 1:4 TO 4:1 RESPECTIVELY, AND POSSESSING IMPROVED SENSITIVITY TO SHORT WAVE-LENGHT SIGNALS, PARTICULARLY EVEN AFTER BEING SUBJECT TO HEAT.

July 16, 1974 GQRO s -u ETAL 3,52,44,128

MAGNETIC RECORDING MEDIUM Filed May 24, 1972 g E E E i 5 c CONTENT o v40 60 80 I00 9 0 coNTENT I00 80 60 40 20 0 FIG 2 2 T (n0 CONTENT 0 20 40so 80 I00 XFe 0 CONTENT I00 so 40 20 0 INVENTORS GORO AKASHI MASAAKIFUJIYAMA TATSUJI KITAMOTO BY SmyQN'wQ YM M ATTORNEYS United StatesPatent 01 fice 3,824,128 Patented July 16, 1974 Int. Cl. H61; N26

US. Cl. 117-235 9 Claims ABSTRACT OF THE DISCLOSURE A magnetic recordingmedium having a magnetic recording layer in which is dispersed in abinder a ferromagnetic powder comprising a mixture of an iron oxidepowder and a chromium dioxide powder in a weight ratio of 1:4 to 4:1respectively, and possessing improved sensitivity to short wavelengthsignals, particularly even after being subjectedto heat.

CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation-in-partapplication of copending application, Ser. No. 864,482 filed Oct. 7,1969, now abandoned.

1. Field of the Invention This invention relates to a magnetic recordingmedium, and particularly, to a composition of a ferromagnetic powder tobe dispersed in a magnetic recording layer of a magnetic recordingmedium.

2. Description of the Prior Art A conventional magnetic recording mediumis composed of a support such as a plastic film, a metalor glassplate,etc., and a magnetic recording layer provided thereon by coating anacicular or granular powder of a ferromagnetic substance of less thanmicrons in size dispersed in a resinous binder anddrying. Severalsubstances, such as iron oxides, chromiurn dioxide, and ferromagneticalloys are hitherto known as such ferromagnetic substances. Among them,iron oxides, such as 'y-Fe O Fe O or modified ones containing a smallamount of cobalt, nickel, chromium, manganese, zinc, etc., have beenmainly used because they can be produced at a low cost and with ease,such as without the necessity for using high temperatures and highpressures.

In recent years, the magnetic recording of short wavelength signalsmeaning a high density magnetic recording in which the minimum recordingwave-length of signal in the conventional magnetic sound recording is 19microns, whereas in the case of video image recording the ability torecord and reproduce signal of wave-length as short as 2 to 3 microns isrequired, has rapidly been developed.

The inventors have found the following five characteristics to berequired for such high density recording.

(1) a high coercive force (2) an excellent surface smoothness of themagnetic recording layer which depends on the dispersibility of theferromagnetic substances (3) a high squareness ratio of the B-H curve(4) heat stability (5) a high packing-density of the ferromagneticpowder in the magnetic recording layer In these requirements,characteristics (1) to (4) are mainly required for a ferromagneticsubstance whereas characteristic (5 is required only for the magneticrecording layer. Ferromagnetic iron oxides conventionally employed formagnetic recording media deteriorate in squareness ratio (Br/Bm) of theB-H curve when the coercive force thereof increases and it is verydifficult to obtain the desired surface property with such materialsbecause of the characteristics of the v-Fe O- modified with cobalt orother metals and with both -Fe O or Fe O modified with cobalt.Accordingly, it is d'tflicult to manufacture a magnetic recordingsubstance suitable for high density recording using conventionalferromagnetic substances such as 'y-Fe O Fe o or the like.

SUMMARY OF THE INVENTION The magnetic recording medium of the presentinvention has a magnetic recording layer comprising a ferromagneticpowder dispersed in a binder and is characterized in that saidferromagnetic powder is a mixture of (1) powdered ferromagnetic ironoxide and/ or iron oxide modified with Co, Ni, Cr, Mn, Zn, andequivalents thereof and (2) powdered ferromagnetic CrO and/or CrOmodified with Te, Sb, Sn, Sn, Fe, and the equivalents thereof employedin a weight ratio of 1:4 to 4:1.

An object of the invention is to provide a ferromagnetic compositionwhich overcomes the above mentioned disadvantages in substancesconventionally used.

Another object of the present invention is to provide a magneticrecording medium having a magnetic recording layer containing the newferromagnetic composition.

A further object of the present invention is to provide a magneticrecording medium having improved sensitivity to shortwave lengthsignals, particularly even after being subjected to heat.

These and other objects of the present invention will be illustratedhereinafter in greater detail.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The drawings aregraphical presentations of the magnetic properties obtainable with themagnetic recording medium of this invention.

DETAILED DESCRIPTION OF THE INVENTION !I he ferromagnetic powderpreferred in the practice of the present invention include 'y-Fe O Fe Oor either of these oxides modified with at least one metal selected fromthe group consisting of Co, Ni, Cr, Mn, Zn and Cu as described in US.Pat. No. 3,117,933; British Pat. Nos. 717,269 and 721,630; and WestGerman Pat. No. 891,- 625, as the ferromagnetic iron oxide, and CrO asdescribed in US. Pat. No. 2,956,955 or CrO modified with at least oneatom selected from the group consisting of Te, Sn, Sb, Ti, V, Cr, Mn,Fe, Co, Ni, N, P, As, Bi, Nb, Ta, Ru and alkali metals as described inUS. Pat. Nos. 2,885,365; 2,923,684; 2,923,685; 3,034,988; 3,068,176 and3,243,260, as the ferromagnetic chromium oxide.

The process of the modification of those substances to be modified, theamount of modifying element used and other conditions necessary toobtain the modified substances, are described in the above-mentionedpatent specifications. 4 E

In any of these ferromagnetic powders, the diameter thereof should beless than about 2;, preferably, about 1O.2,u..

In accordance with the present invention, the above ferromagnetic ironoxide powder and ferromagnetic chromium oxide powder are mixed in aweight ratio of 4:1 to 1:4. This range of the ratio will be explained indetail referring to the drawings (in Example 1).

The above-described powders are dispersed in a solution comprising abinding agent and an organic solvent in order to form a magneticrecording layer (i.e., a layer with magnetic properties). This bindingagent is a high molecular weight compound having film-forming propertiesand, after the formation of a film, having some resistance to abrasion,and capable of forming an adhesive coating composition onto the surfaceof a support or a subbing layer provided on the support.

Specific examples of such binding agents are vinyl chloride-vinylacetate copolymer, epoxy resin, nitrocellulose, polyester resin,chlorinated polyethylene resin, polyvinyl butyral, acrylic acid resin,methacrylic acid resin, amino resin, vinylidene chloride-acrylonitrilecopolymer, phenol resin, urea resin, melamine resin, polyamide resinbutadiene-acrylonitrile copolymer, etc. All of these are publicly knownhigh polymers and may be used independently or in combination byconsidering the properties of the resulting film obtained. These bindingagents are employed in an amount ranging from about 50 to 180 parts byweight, preferably from about 70 to 120 parts by weight for each 300parts by weight of the ferromagnetic powders. When used in an amountoutside this range, the magnetic characteristics of the resultingmagnetic layer will be insufiicient if too much binding agent is used,while the resulting magnetic layer formed will be too fragile and theadhesiveness to the support will deteriorate if too great an amount ofthe ferromagnetic powder is used.

Once the ingredients of the binding agent are selected, an organicsolvent to dissolve the ingredient is then chosen. In selecting theorganic solvent, such solvents which do not dissolve or swell thesupport used must be selected if a high polymer is used as a support.

As specific examples of organic solvents, there are illustrated esterssuch as methyl acetate, ethyl acetate, propyl acetate, butyl acetate,and amyl acetate; hydrocarbons such as toluene, xylene, alcohols such asethanol, propanol, butanol, and amyl alcohol; liquid chlorinatedhydrocarbons such as methylene dichloride, and ethylene dichloride;dimethyl sulfoxide, dirnethyl formamide; ketones such as methyl ethylketone, and methyl isobutyl ketone, etc. One or more of these solventscan be prop erly selected.

Fundamentally, the coating solution for forming the magnetic recordinglayer is that obtained by dispersing the above-described ferromagneticpowders in an organic solution containing the binding agent above. But,since this solution is applied to a support in a subsequent step,additives which are helpful for conducting the coating step rapidly andsmoothly and for forming a coating film which is uniform in compositionand thickness, such as a coating agent capable of improving the coatingproperties so that the coating to a support can be effected rapidly anduniformly, an antifoarning agent for the coating solution and adispersing agent for the ferromagnetic powders, and additives necessaryfor improving the characteristics of the resulting magnetic recordingmedium, such as a lubricant, may be added to the coating solution.

,In addition, it is required further to knead the ingredients of thecoating solution sufliciently in order to obtain a completely uniformcomposition at any part of the resulting coating film. It is necessaryto give the coating solution an appropriate viscosity by adjusting theamount of above-mentioned organic solvent added such that it can beapplied to a support at room temperature and so that it is capable offorming a magnetic layer having the desired thickness, and thatorientation of the ferromagnetic powders conducted in a magnetic fieldis possible immediately after the coating. i

The coating solution may be prepared by charging each of the ingredientsinto a mixing vessel such as a ball mill. Conventional techniques may besatisfactorily employed as to the mixing method, the mixing order or themeans employed for mixing.

The coating solution thus obtained is then applied to the support or asubbing layer provided so as to improve the adhesiveness between thesupport and the magnetic layer. The amount of the coating solutionapplied is so adjusted that the resulting coating layer has sufiicientthickness to exhibit the necessary magnetic characteristics.

As the support for a magnetic recording medium, there are illustratedvarious materials such as metals, e.g., aluminum, high polymers, e.g.,polyethylene terephthalate and cellulose acetate, glass and paper. Theymay be used in the form of tapes, sheets, discs, drums, etc., inaccordance with the expected use or apparatus to which they are applied.

(When the resulting magnetic recording medium is formed into a tape,orientation of the ferromagnetic powders is conducted generallyimmediately after applying the coating solution to a support. Methods oforientation in a magnetic field are described in Japanese PatentPublication Nos. 3,567/53; 21,571/61; 13,931/62; 3,037/63; 19,281/64;23,677/64; 25,245/64; 26,636/64; 26,908/64; 29,471/64; 5,350/65;5,351/65; 23,623/65; 23,624/65; 23,625/65; 23,626/65; 23,945/65;2,065/66; 21,146/67; 15,207/68; 21,251/68; and US. Pat. 2,711,901, andthus, such methods are well known to the art. This orientation in amagnetic field may be replaced by a mechanical orientation. However, themechanical orientation is conducted after the drying of the coatedsubstance.

After conducting the orientation in a magnetic field, the coatedsubstance is immediately dried to maintain the ferromagnetic powders inthe oriented state. Thereafter, any necessary after-treating such asabrasion of the formed surface of the magnetic layer is conducted, andthe resulting substance is properly formed into a desired shape.

The magnetic recording medium in accordance with the present inventionis extremely excellent in surface prop- I erties (i.e., in surfacedurability and the smoothness of the magnetic layer) and the squarenessratio of the B-H curve is likewise excellent. Further, they can bemanufactured more cheaply than in the case of using a magnetic substanceof chromium dioxide alone. They are also characterized by excellentthermal stability.

Comparative Example 1 32 g. of nitrocellulose as a binder, 9 g, ofdibutyl phthalate, 1 g. of castor oil and g. of acicular 'y-Fe O havingan average particle size of 0.6 x 0.1 x 0.1/L were placed in a ball milltogether with 230 cc. of ethyl acetate and the resultant composition wasstirred for 4 days to disperse and to prepare a coating composition.Thereafter, the composition thus obtained was coated on a polyethyleneterephthalate film of 24,11. in thickness and while the magnetic layerwas still wet, the coating was subjected to a magnetic field to orientthe acicular particles in order to provide maximum sensitivity in therecording direction. Then the coating was dried to form a film having adried thickness of 5,. The surface of the coating of the material soobtained was treated using a super calender (a surface calendering roll)and, thereafter, was slit to make a magnetic tape.

The characteristics of the resultant magnetic recording tape included acoercive force of 310 cc. and a square ness ratio of 0.76. The tape hada surface variation of 0.26 1

In testing the reproducing sensitivity of this magnetic recording tapeat a recorded Wavelength of 3a in a video tape recorder capable ofrecording video signals at a relative velocity of 11 m./sec. withrespect to the magnetic head, the output power was found to be lowerthan the desired value by about 2 db.

Comparative Example 2 As a means for improving the squareness ratio andthe surface properties of magnetic recording media appropriate selectionof binders and the addition of suitable dispersing agents, etc. havebeen suggested. Therefore, a magnetic recording medium was prepared witha magnetic iron oxide in combination with one or more resins selectedfrom epoxy resins, acrylic resins, urea resins, cellulose resins, etc.as a binder and one or more surface active agents selected from thequaternary ammonium salts and alkyl pyridinium salts, polyoxyethylenealkylether, polyoxyeth- Comparative Example 3 32 g. of nitrocellulose asa binder, 7 g. of dibutylphthalate, 1 g. of castor oil, and 100 g. of Crhaving a coercive force of about 320 oe. were charged into a ball milltogether with 100 g. of ethyl acetate as a solvent and, after dispersiontreatment for 4 days, the resultant dispersion was coated on apolyethylene terephthalate film of 24a in thickness and dried so as togive a dried thickness of 5p. The coated material so obtained wastreated using a super calender and thereafter, was slit to obtain thedesired magnetic recording tape. After the above coating step and beforethe coating layer was dried, the magnetic medium was subjected to themagnetic orientation treatment of Comparative Example 1, which iscommonly carried out in manufacturing a magnetic recording medium. Theresultant magnetic recording tape possessed a coercive force of 320 oe.and a squareness ratio of 0.90. The tape surface was even, having asurface variation of 0.14

Testing of the resultant tape at a recorded wavelength of 3 in the samemanner as in Comparative Example 1 showed this magnetic tape to be +2db. in output power and higher in reproducing output than that inComparative Example 1 by 4 db. It was therefore suitable for highdensity recording in this respect. However, a decrease of reproducingoutput as a result of heating was observed, which is a characteristic ofCrO The reproducing output after 1 hour at 90 C. was found to have beenlowered to about half of the initial output. Therefore, in case of usingCrO as in the present Comparative Example, should the tape be heatedafter recording it becomes practically impossible to reproduce therecorded signal.

Example 1 32 g. of nitrocellulose as a binder, 8 g. of dibutylphthalate, 1 g. of castor oil and 100 g. of each of ferromagneticpowders as disclosed below were mixed together with 230 cc. of ethylacetate as a solvent in a ball mill for 4 days to prepare a coatingcomposition. In this case, the ferromagnetic powders were a powder ofacicular 'Y-FE203 having an average particle size of 0.6 x 0.1 x 0.1 1.and having a coervice force of 320 ic., a powder of acicular Cr0 havingan average particle size of 0.7 x 0.1 x 0.1 1. and having a coerciveforce of 320 oe. alone, and nine mixtures of both powders in weightratios ranging from 1:9 to 9:1 shown in the drawings were prepared.

Each of the prepared coating compositions was coated on polyethyleneterephthalate films having a thickness of 24 and subjected to magneticorientation treatment before the coating layer was fixed and dried as inthe conventional production thereof.

The characteristics of the magnetic recording tapes so obtained, i.e.,in surface properties and the squareness ratio of the 8-H curve (Br/Bm)were measured. Also the amount of lowering of the reproducing outputcaused by heat were measured. The results shown in FIGS. 1 and 2 wereobtained.

In FIGS. 1 and 2, the absicissas shows 100% of 'y-Fe O powder at theleft end and 100% of CrO powder at the right end and shows the mixingratio, by weight, of both powders in between. The ordinates show thesquareness ratio (Br/Bm) and the surface unevenness (unitsr in FIG. 1and shows the sensitivity (unitzdb) for a signal of 3p. in recordedwavelength in FIG. 2.

In FIG. 1, Curve A shows the squareness ratio and Curves B shows thesurface unevenness. In FIG. 2, Curve C shows the output before heatingand Curve D shows the output after heating.

From the results shown in FIG. 1, it will be understood that the surfaceunevenness, squareness ratio, etc. of the magnetic recording tape wereimproved with an increase in the content of CrO and were abruptlyelevated at the point of 20% in the ratio of Also, from the resultsshown in FIG. 2, the reproducing output measured after recording asignal of 3 in recorded wave-length on the magnetic recording tapefollowed by heating at C. for 1 hour was improved by mixing. That is, anexcellent magnetic recording tape, usable both under ordinarytemperatures and under high temperatures can be obtained by employing aCrO -Fe O ratio ranging from 1/4 to 4/ 1.

Example 2 32 g. of nitrocellulose as the main ingredient of a binder 8g. of dibutyl phthalate, 1 g. of castor oil and 50 g. each of Cr0 and'y-F6 O modified with Co, each possessing a coercive force of 420 oe.were treated in a ball mill for 4 days and, thereafter, were coated on apolyethylene terephthalate film of 24 in thickness and dried so as togive 5 1. of dried thickness. Further, the surface of the coating wastreated by means of a supercalender and was slit to prepare a magneticrecording tape. After coating of the magnetic lacquer, the material wassubjected to the magnetic orientation treatment as described abovebefore the coating layer was dried, as in case of the production of aconventional magnetic medium.

The characteristics of the magnetic recording tape thus obtained were acoercive force of 420 oe. and a squareness ratio of 0.87. The tapepossessed surface variations of 0.15,. In the case of recording a signalof 3,11. in recorded wavelength, the tape showed +2.1 db in reproducingsensitivity and was then suitable for high density recording.

Alternatively, the characteristics of a recording medium composed of -neo, not containing C10 and modified with Co, which was manufactured inthe same manner included a squareness ratio of 0.705, surface Variationsof 023 and --1.2 db in reproducing output.

As shown in the above Examples, the present invention provides theeffect of improving the characteristics of a magnetic recording mediumsuch as the surface smoothness and squareness ratio in the BH curve,which cannot be obtained in case of using only magnetic iron oxide as amagnetic powder for magnetic recording medium and in addition results inanother effect, a decrease in the reduction of output by heating, whichis caused when only magnetic chromium dioxide is employed.

The reason for the deterioration in output power of a magnetic recordingmedium comprising chromium dioxide, when heated, is the lower Curiepoint of the magnetic material itself. Further, the reason why thecombination of iron oxide and chromium dioxide according to the presentinvention lessens such deterioration is believed to be thatmagnetization of the iron oxide itself is not lowered because of itshigh Curie point and the chromium dioxide is remagnetized during coolingto room temperature after heating because of its low coercive force athigh temperature.

Further, by operating in accordance with the present invention, themagnetic recording medium can be manufactured more cheaply by blendingrelatively cheap magnetic iron oxide powder since the manufacturing costof chromium dioxide is high.

A remarkable feature of the present invention is that the effects ofmixing chromium dioxide powder and magnetic iron oxide powder do notvary in proportion to the ratio of the mixed powders, the mixture ofthese materials having proved to be synergistic in effect. That is, asshown in FIG. 1, almost same level of squareness ratio and surfaceproperties are maintained even if a considerable amount of iron oxide isadded to the CrO Further, as shown in FIG. 2, the chromium dioxide isnot significantly lowered in output power before heating until aconsiderable amount of iron oxide has been added. Further, its outputpower after heating reaches a maximum point at almost a ratio of CrO to'y-FeO of 40:60 in the mixture. In both Figures, the expected values areshown by the dotted lines, indicating that such remarkable elfects havenot hitherto been anticipated.

Although the present invention is not to be limited by any theory ofoperation, these effects are believed to depend upon the CrO particles,having good orientation, promoting the orientation of the iron oxideparticles and, further, the thermal stability of iron oxide particlesstabilizing the Cr interposed therein.

What is claimed is:

1. In a magnetic recording medium comprising a support bearing thereonat least one magnetic recording layer comprising ferromagnetic powderdispersed in a binder, the improvement consisting essentially of saidferromagnetic powder comprising a mixture of:

(a) a ferromagnetic iron oxide powder selected from the group consistingof ferromagnetic iron oxide and ferromagnetic iron oxide modified with amodifying amount of at least one member selected from the groupconsisting of cobalt, nickel, chromium, manganese, zinc and copper; and

(b) a ferromagnetic chromium dioxide powder selected from the groupconsisting of a ferromagnetic chromium dioxide and a ferromagneticchromium dioxide modified with a modifying amount of at least one memberselected from the group consisting of tellurium, tin, antimony,titanium, vanadium, chromium, manganese, iron, cobalt, nickel, nitrogen,phosphorus, arsenic, bismuth, niobium, tantalum, ruthenium, or an alkalimetal the weight ratio of said ferromagnetic iron oxide powder to saidferromagnetic chromium dioxide powder being from 1:4 to 4:1.

2. The magnetic recording medium of claim 1 wherein said ferromagneticiron oxide powder is 'y-Fe O or Fe O 3. The magnetic recording medium ofclaim 1 wherein netic chromium dioxide powder are both acicular. saidferromagnetic iron oxide powder and said ferromag- 4. The magneticrecording medium of claim 1 wherein said binding agent is a syntheticresin selected from the group consisting of a vinyl chloride-vinylacetate copolymer, an acrylic resin, a cellulose resin, anepoxy-polyamide resin, a polyurethane resin and combination thereof.

5. The magnetic recording medium of claim 1 wherein said ferromagneticiron oxide powder and said ferromagnetic chromium dioxide powder areboth granular.

6. The magnetic recording medium of claim 2 wherein said ferromagneticchromium dioxide powder is ferromagnetic chromium dioxide.

7. The magnetic recording medium of claim 6 wherein the weight ratio ofsaid ferromagnetic iron oxide powder to said ferromagnetic chromiumdioxide is about 60:40.

8. The magnetic recording medium of claim 1 wherein the diameter of saidferromagnetic iron oxide powder and said ferromagnetic chromium dioxidepowder is less than 2 microns.

9. A magnetic recording medium having excellent surface smoothness,improved squareness ratio and a resistance to output reduction onheating, said magnetic recording medium comprising a support bearingthereon at least one magnetic recording layer which comprisesferromagnetic powder dispersed in a binder therefor, said ferromagneticpowder consisting essentially of a mixture of:

(1) a ferromagnetic iron oxide powder selected from the group consistingof ferromagnetic -Fe O or ferromagnetic Fe O and said ferromagneticmaterials modified with a modifying amount of at least one memberselected from the group consisting of cobalt, nickel, chromium,manganese, zinc and copper; and

(2) a ferromagnetic chromium dioxide powder selected from the groupconsisting of ferromagnetic CrO and ferromagnetic CrO modified with amodifying amount of at least one member selected from the groupconsisting of tellurium, tin, antimony, titaniurn, vanadium, chromium,manganese, iron, cobalt, nickel, nitrogen, phosphorus, arsenic, bismuth,niobium, tantalum, ruthenium, and an alkali metal;

the weight ratio of said ferromagnetic iron oxide powder to saidferromagnetic chromium dioxide powder being from 1:4 to 4:1, whereinsaid ferromagnetic iron oxide powder and said ferromagnetic chromiumdioxide powder are either both acicular or both granular.

References Cited UNITED STATES PATENTS 3,613,100 10/1971 Kaufer et al.117-235 DANIEL E. WYMAN, Primary Examiner A. P. DEMERS, AssistantExaminer US. Cl. X.R.

